What is the workflow for compositing Fire/Smoke with Cycles
Expanding on tobkum's answer, an example workflow using the technique in this tutorial might be:


Setup BI scene (simulation, etc.)
Create a new scene for Cycles, with Link object data
Create Cycles materials, linking to Object instead of Data so BI and cycles materials are not synced:

 
In the BI scene:


Create BI materials, again using Object instead of Data.
Disable shadows for all BI materials except the smoke
Create renderlayers for smoke and smoke shadows using the Shadow pass.
Create renderlayers for reflections and refractions, and remove sky by setting the Horizon color to black.

Setup compositing in Cycles scene:


Create renderlayers for foreground and background objects
Multiply BI shadows with cycles background layer
Alpha over BI smoke over cycles background layer
Alpha over cycles foreground layer over composite
Add or Mix BI reflections and refractions to composite

Render